The present invention relates generally to a high efficiency refrigeration system and more specifically, to a refrigeration system utilizing a bypass path to perform refrigerant de-superheating outside the condenser thereby increasing the overall system efficiency.
FIG. 1 is a block diagram of a conventional refrigeration system, generally denoted at 10. The system includes a compressor 12, a condenser 14, an expansion device 16 and an evaporator 18. These components are connected together via copper tubing such as indicated at 20 to form a closed loop system through which a refrigerant such as R-12, R-22, R-134a, R-407c, R-410a, ammonia, carbon dioxide or natural gas is cycled.
The main steps in the refrigeration cycle are compression of the refrigerant by compressor 12, heat extraction from the refrigerant to the environment by condenser 14, throttling of the refrigerant in the expansion device 16, and heat absorption by the refrigerant from the space being cooled in evaporator 18. This process, sometimes referred to as a vapor-compression refrigeration cycle, is used in air conditioning systems, which cool and dehumidify air in a living space, in a moving vehicle (e.g., automobile, airplane, train, etc.), in refrigerators and in heat pumps.
FIG. 2 shows the temperature-entropy curve for the vapor compression refrigeration cycle illustrated in FIG. 1. The refrigerant exits evaporator 18 as a saturated vapor (Point 1), and is compressed by compressor 12 to a very high pressure. The temperature of the refrigerant also increases during compression, and it leaves the compressor as superheated vapor (Point 2).
A typical condenser comprises a single conduit formed into a serpentine-like shape with a plurality of rows of conduit lying in a spaced parallel relationship. Metal fins or other structures which provide high heat conductivity are usually attached to the serpentine conduit to maximize the transfer of heat between the refrigerant passing through the condenser and the ambient air. As the superheated refrigerant gives up heat in the upstream portion of the condenser, the superheated vapor becomes a saturated vapor (Point 2a), and after losing further heat as it travels through the remainder of condenser 14, the refrigerant exits as saturated liquid (Point 3).
As the saturated liquid refrigerant passes through expansion device 16, its pressure is reduced, and it becomes a liquid-vapor mixture comprised of approximately 20% vapor and 80% liquid. Also, its temperature drops below the temperature of the ambient air (Point 4 in FIG. 2).
Evaporator 18 physically resembles the serpentine-shaped conduit of the condenser. Air to be cooled is exposed to the surface of the evaporator where heat is transferred to the refrigerant. As the refrigerant absorbs heat in evaporator 18, it becomes a saturated or slightly superheated vapor at the suction pressure of the compressor and reenters the compressor thereby completing the cycle (Point 1 in FIG. 2).
FIG. 3 shows the temperature-entropy curve for the vapor compression refrigeration cycle, in which the de-superheating process in the condenser is indicated explicitly. The pressure of the discharge vapor from the compressor has to be raised such that the phase-change temperature (known as the saturation temperature) at the saturation pressure can be large enough to reject heat at the condenser. This requires that the discharge vapor from the compressor is superheated as the entropy increases slightly over the compressor as shown in FIG. 2. Typically one-third of a condenser is utilized for the de-superheating process in most air-conditioning and refrigeration systems.
This is a source of significant inefficiency in conventional refrigeration systems as the condenser must be larger and more costly than needed for the heat transfer function involving the phase-change of the refrigerant. Conversely, for a condenser of a given size, if the first one-third does not need to be devoted to de-superheating, greater subcooling could be achieved.
An additional benefit which could be achieved by performing the de-superheating step outside the condenser would be an improved energy-efficiency ratio (EER). This is defined as Qv/Wc, where Qv is the heat absorption by the evaporator of the system and Wc is the work done by the compressor. By increasing subcooling for a given size condenser, a greater quantity of liquid in the refrigerant would enter the evaporator. This would increase the cooling capacity Qv, thus the EER would also increase. Furthermore, as the condenser becomes more efficient, the condenser pressure decreases, reducing the required pressure lift across the compressor, thereby reducing the compressor work and accordingly increasing the EER.
FIG. 4 illustrates a modified temperature-entropy curve showing what would happen if the de-superheating step could be performed between the compressor and the condenser. Heat would be removed from the vapor discharged from the compressor, reducing the temperature of the vapor substantially while the saturation pressure is almost unchanged. Consequently, the vapor from the compressor could enter the condenser at or close to its saturation temperature and pressure. This is illustrated in the modified temperature-entropy curve of FIG. 4 between points 2c and 2a. Up to now, however, no suitably cost effective technique has been available to eliminate the need for de-superheating in the condenser.
Therefore, a need clearly exists for a cost-effective way to achieve de-superheating at the inlet side of the condenser. The present invention seeks to meet this need.
According to the present invention, the de-superheating step is performed on the inlet side of the condenser, rather than in the condenser. To achieve this, a portion of liquid refrigerant exiting from the condenser is diverted into a bypass line from which it is re-injected into the primary refrigerant path at a location between the evaporator outlet and compressor inlet. In the bypass line, a secondary expansion valve is used to throttle the diverted liquid refrigerant from the condenser, thus decreasing the temperature substantially below the condenser outlet temperature.
The cooled refrigerant exiting the secondary expansion valve then passes through a heat exchanger which is thermally coupled to the primary refrigerant line between the compressor outlet and the condenser inlet. The heat exchanger removes heat from the refrigerant vapor exiting from the compressor, thus reducing its temperature. As a result, the refrigerant enters the condenser at or near its saturation temperature, and no portion of the condenser needs to be devoted to de-superheating.
Because the refrigerant pressure in the bypass line at the outlet of the heat exchanger is greater than the pressure at the evaporator outlet, a pressure differential compensating device is used to couple the outlet of the bypass line to the primary refrigerant line. The pressure differential compensating device can be either a vacuum generating device or a pressure-reducing device.
According to a first aspect of the invention, there is provided a refrigeration system including refrigerant compressing means, refrigerant condensing means, expansion means and evaporation means connected together to form a closed loop system with a refrigerant circulating therein, and a bypass line connected between the outlet of the condensing means and the inlet of the compressing means, the bypass line including a secondary expansion means, heat exchanging means to remove heat from the discharge vapor from the compressor between the outlet of the compressing means and an inlet of the condensing means, and a pressure differential accommodating means for mixing two vapors at two different pressures connecting the outlets of the evaporation means and the heat exchanging means to an inlet of the compressing means.
According to a second aspect of the invention, there is provided a refrigeration system comprised of a primary refrigerant path including a compressor, a condenser, a primary expansion device, and an evaporator connected together to form a closed loop system with a refrigerant circulating therein, and a bypass line connected between the outlet of the condenser and the inlet of the compressor, the bypass line including a heat exchanger thermally coupled to the primary refrigerant path between the compressor outlet and the condenser inlet to remove heat from the discharge vapor from the compressor, and a pressure differential accommodating device for mixing two vapors at two different pressures connecting the outlets of the evaporator and the heat exchanger to an inlet of the compressor.
Further according to the second aspect of the invention, the pressure differential accommodating means is a vacuum generating device with no moving parts such as a venturi tube, or a so-called xe2x80x9cvortex tubexe2x80x9d which is conventionally used to create two fluid steams of differing temperature from a single high pressure input stream. (Such a vortex generator is the subject of a copending U.S. provisional patent application entitled USE OF A VORTEX GENERATOR TO GENERATE VACUUM, Serial No. 60/356,059 filed in the names of Young Cho, Cheolho Bai, and Joong-Hyoung Lee on Feb. 11, 2002, the contents of which are hereby incorporated by reference.)
Further according to the second aspect of the invention, the pressure differential accommodating means is a pressure reducing device with no moving parts such as a capillary tube, an orifice, a valve, or a porous plug. The pressure reducing device is used in the bypass line which is maintained at a higher pressure than the evaporator. The pressure reducing device reduces the high pressure at the bypass line to the evaporator pressure so that two vapors can be mixed.
According to a third aspect of the invention, there is provided a method of increasing the efficiency of a refrigeration system comprised of a primary refrigerant path including a compressor, a condenser, a primary expansion device, and an evaporator connected together to form a closed loop system with a refrigerant circulating therein, the method comprising the steps of bypassing a portion of the refrigerant exiting the condenser into a secondary refrigerant line, passing the bypassed refrigerant through a heat exchanger thermally coupled to the primary refrigerant path between the compressor outlet and the condenser inlet to remove heat from the discharge vapor from the compressor, and passing the refrigerant exiting the heat exchanger and the refrigerant exiting the evaporator through a pressure differential accommodating device means that mixes two vapors at different pressures and feeding the refrigerant exiting the pressure differential accommodating device to an inlet of the compressor.
Providing a bypass path for performing de-superheating makes the condenser more efficient thereby reducing the condenser pressure, a phenomenon which decreases the pressure lift at compressor, and thus reduces the compressor work. Correspondingly, because de-superheating does not have to be done inside the condenser, the condenser becomes more efficient, and subcooling at the end of the condenser is increased. This increases the amount of liquid refrigerant after the throttling process through the main expansion valve. Thus, the heat absorption at the evaporator (often referred as the cooling capacity) increases.
The above-described benefits of the de-superheating bypass are achieved with diversion of 10-15% of the liquid refrigerant outflow from the condenser. At this level, reduced compressor work and increased cooling capacity are achieved. Since the EER (energy efficiency ratio) is defined as the ratio of the cooling capacity to compressor work, this increases the EER.
According to a fourth aspect of the invention, when more than 15%, for example, 30%, of the liquid refrigerant from the condenser is diverted to the bypass path, the cooling capacity is reduced due to the substantial decrease in the refrigerant mass flow rate circulating through the evaporator. By use of an adjustable valve in the bypass path, the bypass mass flow rate, and thus, the cooling capacity, can be varied according to the thermal load, whereby it is possible to operate an air conditioning or refrigeration system without frequent, highly energy-inefficient, ON-OFF operations of the compressor. This results in improved long-term seasonal energy efficiency ratio (SEER).
According to a fifth aspect of the invention, multiple evaporators can be employed, e.g., in a zoned cooling system. Thus, several small evaporators could be provided for separate rooms, with one condenser and one compressor. When all the rooms require cooling, the system can be operated with a 10% bypass rate to provide the maximum cooling capacity and the maximum efficiency. If the thermal load decreases, as when fewer rooms need to be cooled, the bypass rate can be increased to reduce the cooling capacity without the need to cycle the compressor on and off. This is quite beneficial because the repeated ON-OFF cycling of the compressor is a very energy-inefficient process.
The concepts of this invention are applicable to conventional single-refrigerant systems, and also to mixed-refrigerant systems using a combination of refrigerants selected to provide the desired combination of thermal and flammability characteristics. Such mixed-refrigerant systems may also include regenerative features which provide higher evaporator efficiency by increasing the percentage of liquid in the refrigerant as it enters the evaporator. Regenerative mixed refrigerant systems are disclosed, for example, in our U.S. Pat. No. 6,250,086 and 6,293,108, the contents of which are hereby incorporated by reference.
It is accordingly an object of this invention to provide an apparatus and method that eliminates the need for de-superheating to take place in the condenser of a refrigeration system.
It is also an object of the invention to increase the efficiency of known refrigeration systems by providing a cost-effective way of reducing the temperature and pressure of the discharge vapor from the compressor.
It is another object of the invention to increase the cooling capacity and EER of known refrigeration systems by providing a cost-effective way of reducing the temperature and pressure of the discharge vapor from the compressor.
A related object of the invention to allow use of smaller condensers in known refrigeration systems by providing a cost-effective way of reducing the temperature and pressure of the discharge vapor from the compressor.
An additional object of the invention is to provide a way of reducing the temperature and pressure of the discharge vapor from the compressor, which may be used in single-refrigerant systems and also in mixed-refrigerant systems, with and without regenerative features.
An additional object of the invention is to provide an improved refrigeration system in which the evaporator is connected to a substantially low pressure created by a vacuum-generating device thereby boosting the evaporator capacity.
An additional object of the invention is to provide an improved refrigeration system in which the mixing two different pressures using a vacuum generating device increases the suction pressure of the compressor, whereby the required pressure rise over the compressor is reduced, which, in turn, reduces the compressor work and increases the EER.
An additional object of the invention is to provide an improved refrigeration system in which the mixing two different pressure vapors are carried out using a vacuum generating device so that the pressure at the bypass line can be maintained at a higher pressure than the evaporator pressure.
An additional object of the invention is to provide an improved refrigeration system in which the mixing two different pressure vapors are carried out using a pressure-reducing device so that the pressure at the bypass line can be maintained at a higher pressure than the evaporator pressure.
Yet another object of the invention is to provide an improved refrigeration system in which de-superheating is performed outside the condenser in a bypass path to which refrigerant from the condenser outlet is diverted, into a bypass path, and in which the quantity of refrigerant diverted is controlled such that the cooling capacity can be adjusted to meet varying thermal requirements, whereby the system can be operated without the need for energy-inefficient repeated on and off cycling of the compressor.